1. Field
The present invention relates to objective designs within an inspection apparatus usable, for example, in the manufacture of devices by lithographic techniques.
2. Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., comprising part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In order to monitor the lithographic process, parameters of the patterned substrate are measured. Parameters may include, for example, the overlay error between two layers formed in or on the patterned substrate and critical line width of developed photosensitive resist. This measurement may be performed on a product substrate and/or on a dedicated metrology target. There are various techniques for making measurements of the microscopic structures formed in lithographic processes, including the use of scanning electron microscopes and various specialized tools. A fast and non-destructive form of specialized inspection tool is a scatterometer in which a beam of radiation is directed onto a target on the surface of the substrate and properties of the scattered or reflected beam are measured. By comparing the properties of the beam before and after it has been reflected or scattered by the substrate, the properties of the substrate can be determined. This can be done, for example, by comparing the reflected beam with data stored in a library of known measurements associated with known substrate properties. Two main types of scatterometer are known. Spectroscopic scatterometers direct a broadband radiation beam onto the substrate and measure the spectrum (intensity as a function of wavelength) of the radiation scattered into a particular narrow angular range. Angularly resolved scatterometers use a monochromatic radiation beam and measure the intensity of the scattered radiation as a function of angle.
With the further shrinking of resist thickness and the introduction of more complicated lithographic stacks (e.g., stacks configured for double exposure), critical dimension (CD) and overlay (OV) metrology performance will also have to improve to monitor the lithographic processes. One way to improve CD metrology performance is to reduce the wavelength of the measurement radiation, for example, ultraviolet radiation. However, OV metrology often requires radiation having near-infrared wavelengths to view overlay targets through different process layers. Accordingly, there is a need for inspection apparatuses, such as scatterometers, that can operate within a spectral band from about 200 nm to about 850 nm to make both CD and OV measurements without compromising optical performance regarding, for example, field of view, pupil aberrations, over all transmission, polarization properties (induced ellipticity), and transmission uniformity. Reduced optical performance can lead to reduced accuracy of the OV and CD measurements, as well as reduced system productivity.
Another way to improve CD and OV metrology performance is to improve the objectives used with inspection apparatuses. There are two types of objectives generally used for scatterometry applications: refractive objectives and catadioptric objectives. One disadvantage of previous refractive objectives is that the working distance is relatively small. For example, the working distance is generally less than 0.35 mm when the numerical aperture (NA) equals about 0.95. Another disadvantage of previous refractive objectives is that the operational wavelength spectral range is limited to about 450-700 nm. Further, previous refractive objectives only have plan apochromatic aberration correction.
One disadvantage of previous catadioptric objectives is that they induce field curvature—previous catadioptric objectives typically have a large Petzval sum that is far from zero and must be corrected for. Previous catadioptric objectives also suffer from obscuration that decreases image contrast. Catadioptric objectives for an inspection apparatus such as a scatterometer must also be corrected for pupil aberrations. Pupil aberrations in the previous catadioptric objectives are large because of large Petzval curvature and pupil size.